Derivative instruments (Summary of Derivatives Outstanding) (Details)	Jun. 30, 2020 bbl MMBTU $ / bbl $ / BTU
Oil swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	1,026
Weighted average fixed price (in usd per unit) | $ / bbl	50.56
Oil swaps | Derivative Maturing In 2021 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	689
Weighted average fixed price (in usd per unit) | $ / bbl	46.24
Oil roll swaps | Derivative Maturing In 2020 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	180
Weighted average fixed price (in usd per unit) | $ / bbl	0.30
Oil roll swaps | Derivative Maturing In 2021 | Crude oil derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | bbl	150
Weighted average fixed price (in usd per unit) | $ / bbl	0.30
Natural gas swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | MMBTU	3,000
Weighted average fixed price (in usd per unit) | $ / BTU	2.75
Natural gas basis swaps | Derivative Maturing In 2020 | Natural gas derivatives
Derivative [Line Items]
Volume (in MBbl, MMBtu, or Gallon) | MMBTU	3,000
Weighted average fixed price (in usd per unit) | $ / BTU	0.46